This invention relates to thermoplastic resin compositions and methods of using them and more particularly relates to improved blow moldable or thermoformable polycarbonate/ABS compositions.
More specifically, this invention relates to blow molding compositions comprising a blend of carbonate polymers with (1) graft copolymers of butadiene or a butadiene-vinyl aromatic hydrocarbon copolymer with a mixture of acrylonitrile and a vinyl aromatic hydrocarbon containing less than 40% rubber and (2) a melt strength enhancing agent having a Tg less than 0.degree. C. and greater than 40% rubber made from a core/shell graft copolymers of butadiene or butadiene-vinyl aromatic hydrocarbon or butadiene-vinyl aromatic hydrocarbon-alkyl acrylate with optionally a second phase of vinyl aromatic hydrocarbon and a outer shell of alkyl (meth)acrylate.
Although polycarbonate/ABS compositions have been found to be thermoplastically moldable under a broad range of injection molding conditions, only select polycarbonate blends are suitable for blow molding. This is due to the unique requirement of thermoplastic resin for blow molding operations.
In the conventional blow molding operation, as taught in U.S. Pat. No. 4,652,602 and U.S. Pat. No. 4,474,999, a tube or parison of the heat softened thermoplastic blend may be extruded vertically downward into a mold. The extrudate is then pressed unto the mold surfaces with a pressurized gas flow (usually air or inert gas), shaping the heat softened resin. As appreciated by those skilled in the art, the successful molding of a given thermoplastic resin is dependent upon a number of factors, including the characteristics and physical properties of the heat softened resin. The length and diameter of the tube and the quantity of material forming the tube are limiting factors in determining the size and wall thickness of the object that can be molded by this process. The fluidity of the melt obtained from polycarbonate/ABS blends, or the lack of melt strength as well as the paucity of extrudate swelling, serve to limit PC/ABS resins in blow molding applications to relatively small, thin walled parts. The above factors alone are of considerable importance in the successful blow molding of any resin, particularly in regard to the molding of large articles.
It is known from Japanese patent 58/59258 that resin compositions with good weldline strength can be obtained from blends of linear polycarbonate resins, acrylonitrile-butadiene-styrene resins (ABS), and rubbery graft copolymer resins (MBS). However, this reference does not suggest the advantage of controlling rubber placement and in fact is attempting to modify the ABS phase to improve the weldline by utilizing MBS core/shell resins containing 30-50% rubber with styrene in the outer shell.
It is further known from U.S. Pat. No. 4,677,162 that a moldable blend of both linear or branched polycarbonate resins (PC), acrylonitrile-butadiene-styrene resins (ABS), and rubbery graft copolymers (MBS) is useful to form articles with good impact and low gloss. However, this reference utilizes only ABS resins with low AN content and does not suggest the advantage of controlling rubber placement by altering the solubility parameter and/or polarity of the ABS resin by varying the acrylonitrile content such that the addition of a second rubbery graft copolymer will be located at the PC interphase or in the PC phase.